Multipole mass filter having means for applying a voltage gradient between diametrically opposite electrodes



May 23, 1967 W. M. BRUBAKER ETAL MULTIPOLE MASS FILTER HAVING MEANS FOR APPLYING A VOLTAGE GRADIENT BETWEEN DIAMETRICALLY OPPOSITE ELECTRODES Filed May 13, 1963 3 Sheets-Sheet 1 V my z 21 40 A? f a M 4 M) y 1967 w. M. BRUBAKER ETAL 3,321,523

MULTIPOLE MASS FILTER HAVING MEANS FOR APPLYING A VOLTAGE GRADIENT BETWEEN DIAMETRICALLY OPPOSITE ELECTRODES Filed May 13, 1963 3 Sheets-Sheet Z (via/2f 4-7 M45 f a! Mame/w May 23, 1967 w. M. BRUBAKER ETAL 3 MULTIPOLE MASS FILTER HAVING MEANS FOR APPLYING A VOLTAGE GRADIENT BETWEEN DIAMETRICALILY OPPOSITE ELECTRODES Filed May 13,

5 Sheets-Sheet 3 Q J T/MHQ KT v VL Q QR United States Patent Illinois Filed May 13, 1963, Ser. No. 279,724 14 Claims. (Cl. 25041.9)

This invention relates to multipole mass filters, and provides an improved arrangement in the application of voltages to the electrodes in mass filters to enhance their operating capabilities.

The basic function of any mass filter is to separate or selectively pass ions having different ratios of mass to charge (M/e). The multipole mass filter accomplishes this in a unique way, without a magnet, by utilizing the motion of charged particles in a multipole electric field having both alternating and static components.

In a typical multipole mass filter, such as that described in US. Patent No. 2,939,952, four elongated continuous electrodes in the form of parallel cylindrical rods are arranged symmetrically about a central axis. The rods are electrically connected in pairs, opposing rods being connected together. If Z denotes the central longitudinal axis of the rods, then one pair of rods lie with their centers in an X-Z plane, and the other pair have their centers in a Y-Z plane, the three axes (X, Y, and Z) being mutually perpendicular according to the convention of rectangular cartesian coordinate system. Both A.C. and DC. voltages are applied to the rods. Ions or charged particles are introduced at one end of the filter and travel generally down the axis of the filter. In transversing the filter, ions of diiferent M e are separated so that only ions of a selected M/e have stable trajectories and emerge from the outlet end of the filter to reach and charge an ion collector, which is connected to a current indicator. Those charged parti cles which are outside the selected M/e range have unstable trajectories and impinge on the field-generating electrodes and thus are neutralized and removed. Ion selection is controlled by varying the voltage levels on the electrodes or by varying the frequency of the A.C. voltages.

The multipole mass filter is potentially very useful for upper atmosphere research, as an analytical device in a satellite vehicle or the like, where the ion source for the filter is the space surrounding the satellite. For such applications, the filter must have high sensitivity, high resolving power, and must consume as little electrical power as possible. As the mass filter is usually used, the A.C. voltage applied to the pairs of electrodes is usually balanced about a common point, say ground, and so is the DC. voltage. When this is done, there is a region along the longitudinal or Z axis of the mass filter where the electric fields are Zero. Under these conditions, all ions entering the mass filter on the axis, and moving parallel to the axis, stay on the axis and reach the collector, whether they have the selected M/e or not. Those ions with M/e outside the selected range, i.e., those ions whose trajectories are unstable, and which enter the mass filter very near and parallel to the axis with small components of radial velocity, have paths which diverge slowly from the axis, and eventually strike the electrodes, provided the apparatus is long enough.

This invention provides a mass filter which removes the region of zero electric fields from the longitudinal axis of the apparatus so that there is no straight path through the filter on which an ion can pass and be undefiected. This increases the radial acceleration for ions with unstab'le trajectories which are near the longitudinal axis, and

3,321,623 Patented May 23, 1967 causes them to reach the rods sooner. It also prevents ions outside the selected M/e range from reaching the collector.

The advantages of this invention are that it increases the resolving power of a mass filter, and effects a substantial power saving by providing the same or better performance than that obtained from a comparable filter which is longer.

In accordance with this invention, the multipole mass filter for selectively detecting charged particles includes a plurality of elongated and substantially parallel electrodes symmetrically disposed about a central axis. Means are provided for applying an A.C. voltage to the electrodes to create an alternating multipole electric field component. Means are also provided for applying a DC. voltage to the electrodes to create a static multipole field component, and further means are provided for imposing a voltage difierence between at least two diametrically opposed electrodes. The imposed voltage difference can be either A.C. or DC, and its function is to eliminate the zero electric field previously existing in prior mass filters along the longitudinal axis of the filter.

A typical multipole mass filter includes four elongated parallel rods symmetrically disposed about a central longitudinal axis. Diametrically opposed electrodes are electrically connected in pairs. An A.C. voltage balanced to a common point, say, ground, is applied across the electrically connected pairs of electrodes, and so is a DC. voltage, which is also balanced to the common point.

Thus, in the typical mass filter, equal but opposite A.C..

and DC. voltages are applied to the electrically connected pairs of electrodes. As indicated previously, this arrangement results in a region of zero electric fields along the longitudinal axis of the apparatus. This condition is avoided in accordance with this invention by applying perturbing voltages which unbalance the previously symmetrically opposed electrical fields. Preferably, the perturbing voltages are applied symmetrically to avoid inducing spurious current in the collector circuit, either when the instrument is observing one peak, or when it is scanning a spectrum. This desired result is obtained if the perturbing potentials are either constant in time, or if they scan with the normal, applied potentials. Preferably, the perturbing voltages are a constant fraction of the total voltage applied to the electrodes.

In one form, the perturbing voltages of equal but opposite polarity are app-lied to diametrically opposed electrodes, i.e., those electrodes electrically connected together. This is accomplished by the addition of a small (relative to the overall voltage applied to the electrodes) DC. or A.C. potential of one polarity to one of a pair of electrically connected electrodes, and the subtraction of an identical DC. or A.C. potential from the other electrode of the pair. This imposes an additional gradient across the mass filter, and the gradient extends from one of the electrodes in the affected pair to the other. This moves the line of zero field from the axis of the instrument for the DC. or the AC. component, whichever is applied as a perturbant, and thus eliminates the coexistence of the regions of zero A.C. and DC. fields.

Alternatively, the desired unbalance is obtained by the use of an additional pair of auxiliary electrodes. One of the auxiliary electrodes is disposed between an adjacent pair of the conventional or primary electrodes in the mass filter, and the other auxiliary electrode is disposed between another pair of adjacent electrodes. Preferably, the auxiliary electrodes are diametrically opposed with respect to each other. The auxiliary electrodes are energized with either A.C. or DC. voltage, preferably with a voltage that is a constant fraction of the total voltage applied to the primary electrodes. It is further preferred that O the voltages applied to the auxiliary electrodes be balanced about the same common point as the D.C. and A.C. voltages applied to the primary electrodes.

These and other aspects of the invention will be more fully understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 is 'a schematic perspective of an array of four electrodes and one circuit for applying a perturbing D.C. voltage to them;

FIG. 2 is a schematic longitudinal sectional view of the electrodes of FIG. I mounted in a mass filter;

FIG. 3 is a schematic circuit diagram of an alternate embodiment for applying a perturbing D.C. voltage to the electrodes in a mass filter;

FIG. 4 is a schematic diagram of a circuit for applying a perturbing A.C. voltage to the electrodes in a mass filter; and

FIG. 5 is a schematic diagram of a circuit for applying either A.C. or D.C. perturbing voltages to a pair of auxiliary electrodes in a mass filter.

Referring to FIG. 1, an electrode array includes an upper electrode 11, a left electrode 12, a right electrode 13, and a lower electrode 14 (as viewed in FIG. 1). The electrodes are cylindrical rods of identical diameter and length, and are disposed to be parallel, co-extensive, and symmetrically located about a longitudinal central axis Z. The longitudinal axes of the upper and lower electrodes lie in a plane defined by the Z axis and a Y axis. The longitudinal axes of the left and right electrodes lie in a plane defined by the Z axis and an X axis. The X, Y, and Z axes are mutually perpendicular.

The upper and lower electrodes are connected through a blocking capacitor 16 to one end of the secondary winding 18 of a transformer 19. Winding 18 is center-tapped to a common reference point, say ground. The primary winding 20 of the transformer is connected to an adjustable radio frequency generator. The other end of the secondary winding is connected between a second blocking condenser 22 and a third blocking condenser 23. The opposite sides of the second and third blocking condensers are connected, respectively, through leads 24 and 25 to the right and left electrodes. The positive terminal of a first main D.C. source 27 is connected through a lead 28 to the negative terminal of a first perturbing D.C. source 30 and to the positive terminal of a second D.C. perturbing source 32. The negative terminal of the main positive D.C. source 27 is grounded. The positive terminal of the first perturbing D.C. source is connected through an induct-or 34 to the right electrode. The negative terminal of the second D.C. perturbing source is connected through an inductor 36 to the left electrode. The first and second D.C. perturbing sources are equal in value, but are applied in opposite polarity to the left and right electrodes.

The negative terminal of a main negative D.C. source 38 is connected through an inductor 40 to the upper and lower electrodes. The positive terminal of the D.C. source 38 is connected to the common reference point, or ground. Thus, using ground as a reference point, the main D.C. voltage applied to the left and right electrodes is indicated in FIG. 1 as +V However, imposed on this voltage is a +VPERTURBING (+V D.C. voltage with respect to ground on the right electrode and a VPERTURBING voltage respect to ground on the left electrode.

A V voltage is applied to the upper and lower electrodes, and is equal in magnitude to +V The alternating voltage to the left and right electrodes may be written V cos wt, which is equal to V cos (wf1r), which is the A.C. voltage applied to the upper and lower electrodes. Thus, with the circuit shown in FIG. 1, equal but opposite A.C. and D.C. voltages are applied to the electrically connected pairs of electrodes. However, the positive voltage on the left electrode is decreased by a relatively small increment, and the positive voltage applied to the right electrode is increased by an equal amount. This produces a small D.C. electrical field gradient between the right and left electrodes, and eliminates the co-existence of the region of zero A.C. and D.C. fields, such as occurs in conventional electrode arrangements for multipole mass filters.

As shown in FIG. 2, the electrode assembly of FIG. 1 is disposed within an elongated cylindrical metal housing 42, with each of the electrodes mounted on respective electrical insulators 43 secured to the interior wall of the housing.

A conductive plate 44 is mounted across the inlet end of the housing, and has a centrally located inlet aperture 45 which forms the ion entrance for the filter. A second conductive plate 46 is mounted adjacent the outlet end of the electrode within the housing, and it has a central circular outlet aperture 47 which serves as the ion exit for the filter. The outlet end of the housing is closed by conductive rear wall 48. An electrically insulating support 49 is mounted on the interior of the rear wall, and an ion collector electrode 50 is mounted on the support opposite the ion exit aperture. Ions of the chosen mass-to-charge ratio enter the entrance aperture, traverse the filter, impinge on the collector, and electrically charge it. The ion current is measured by convention-a1 measuring circuit 52 connected between the ion collector and the ground. The conductive housing, aperture plates, and the rear wall are all at ground potential.

When used in the laboratory, the housing is evacuated and an ion source is mounted over the entrance aperture. For upper atmosphere research, the ion entrance aperture is open and the vacuum inside the device is the vacuum of space.

When operating the mass filter, as shown in FIGS. 1 and 2, the voltages are applied to the electrodes as generally indicated in FIG. 1. The A.C. component of the voltage is the same for electrodes connected in pairs, and so is the D.C. voltage except for the small D.C. increments superimposed by the perturbing voltage sources, which cause a relatively weak D.C. electrical field gradient between the left and right electrode-s. This moves the line of zero field from the axis of the instrument for the D.C. component, and therefore all ions entering the filter, whether on the longitudinal axis or not, are subjected to either an A.C. or a D.C. electrical field gradient, or both. This increases the resolving power of the mass filter and permits a saving in power in that the length of the filter may be made shorter than that previously required when the region of zero of A.C. and D.C. fields were co-existent on the longitudinal axes of the electrode array.

In FIG. 3 an unbalancing D.C. potential is applied to the left and right electrodes in a slightly different manner from that shown in FIG. 1. The positive terminal of a main positive D.C. source 52 is connected through a lead 53 between one end of the first inductor 34 and a first resistor 54. The main positive D.C. source 52 is greater by an amount AV than the voltage of the main negative D.C. source 38, which has its negative terminal connected through the inductor 40 to the upper and lower electrodes. The end of the first resistor 54 opposite from lead 53 is connected through a relatively large resistor 56 to ground and through inductor 36 to the left electrode. The flow of current through resistors 54 and 56 is such that the voltage drop through resistor 54 is equal to ZAV and therefore the voltage applied to the left electrode is equal but opposite in sign to the voltage at 38 minus the value AV FIG. 4 is a schematic diagram of a circuit for unbalancing the A.C. electrical field within the electrodes. A positive terminal of a first main D.C. source 60 is connected through a lead 61 to common ends of first and second inductors 62 and 63, the opposite ends of which are connected to the right and left electrodes, respectively. The negative terminal of a second main D.C.

source 64 is connected through lead 65 and an inductor 66 to the upper and lower electrodes. One end of a secondary winding 67 of a transformer 68 is connected through a lead 69 and a blocking capacitor 70 to the upper and lower electrodes. The secondary winding is connected to ground at a point slightly closer to its end connected to the upper and lower electrodes than its other end, which is connected through a lead 72 to one side of a capacitor 74. The other side of the capacitor 74 is connected to the right electrode and to one end of an impedance 76, the other end of which is connected through a lead 77 to the left electrode. The unbalanced position of the grounded secondary winding develops a slightly larger A.C. voltage at the left end of the winding electrodes than at the right. The A.C. voltage developed at the left end of the winding 67 is expressed in FIG. 4 mathematically as V (1+A) cos wt. The voltage applied across the upper and lower electrodes and ground is expressed in FIG. 4 mathematically as V cos (wt1r). The value of the impedance 76 is such that the A.C. voltage dropped across it is ZAV Thus, the voltage applied to the left electrode (with-respect to ground) is V (l-A) cos wt.

Referring to FIG. 5, one end of a secondary winding 80 of a transformer 81 is connected to the positive terminal of a first D.C. source 82, the negative terminal of which is connected through lead 83 to the upper and lower electrodes. The secondary winding is center-tapped to ground, and its opposite end is connected to the negative terminal of a second D.C. source 84, the positive terminal of which is connected through a lead 85 to the left and right electrodes. The voltages supplied by the first and second D.C. sources are equal so that the voltage applied to the left and right electrodes may be expressed mathematically as V -I-V cos wt, and the voltage applied to the upper and lower electrodes as -(V +V cos wt).

A first elongated auxiliary electrode 86 in the form of a cylindrical rod is disposed in a plane passing through the longitudinal axes of the upper and right electrodes, and is equidistant from them. A second auxiliary electrode 87 identical with the first auxiliary electrode is disposed in the plane passing through the longitudinal axes of the left and lower electrodes, and is equidistant from them. The auxiliary electrodes may run the entire length of the filter, or they may be disposed in the vicinity of the entrance only.

One terminal of a first source 88 of perturbing voltage is connected to the first auxiliary electrode, and the other terminal is connected to ground. One terminal of a second source 89 of perturbing voltage is connected to the second auxiliary electrode, and the other terminal is connected to ground. The perturbing sources 88 and 89 may be either A.C. or D.C., but in either event, they are equal in magnitude. If D.C., the sources are of opposite polarity,'-and, if A.C., they are of opposite phase, so that either the D.C. or A.C. region of zero field is displaced from the longitudinal central axis of the electrodes.

We claim:

1. A multipole mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetric-ally disposed about a central axis, means for applying an A.C. voltage to the electrodes to create an alternating multipole electric field component, means for applying a D.C. voltage to the electrodes to create a static multipole field component, means for providing a voltage difference between at least two diametrically opposite electrodes, and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined massto-charge ratio are selectively detected;

2. A multipole mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetrically disposed about a central axis, means for applying equal and 0pposite A.C. voltages to the electrodes to create an alternating multipole electric field component, means for applying equal and opposite D.C. voltages to the electrodes to create a static multipole field component, the D.C. and A.C. voltages being balanced about a common point, means for providing a voltage dilference between at least two diametrically opposite electrodes, the voltage difference being balanced about the common point and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

3. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetrically spaced about a central axis, means for applying a D.C. voltage to the primary electrodes to create a static multipole electric field component between the electrodes, means for applying an A.C. voltage to the electrodes to create an alternating multipole electric field component between the electrodes, means for providing a D.C. voltage diflerence between two diametrically opposite electrodes and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined massto-charge ratio are selectively detected.

4. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetrically spaced about a central axis, means for applying equal and opposite D.C. voltages to the primary electrodes to create a static multipole electric field component between the electrodes, means for applying equal and opposite A.C. voltages to the electrodes to create an alternating multipole electric field component between the electrodes, means for providing a D.C. voltage difference between two diametrically opposite electrodes, the D.C. and AC. voltages and the D.C. voltage difference being balanced about a common point and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

5. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetrically spaced about a central axis, means for applying a D.C. voltage to the electrodes to create a static multipole electric field component between the electrodes, means for applying an A.C. voltage to the electrodes to create an alternating multipole electric field component between the electrodes, means for providing an A.C. volt-age difference between at least two diametrically opposite electrodes of similar polarity and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

6. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, a plurality of substantially parallel electrodes symmetrically spaced about a central axis, means for applying equal and opposite D.C. volttion substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-tocharge ratio are selectively detected.

7. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, at least four primary electrodes symmetrically disposed about a central axis, means for applying a DC. voltage to the primary electrodes to create a static multipole electric field component between the primary electrodes, means for applying an A.C. voltage to the primary electrodes to create an alternating multipole electric field component between the primary electrodes, a first auxiliary electrode disposed between a first pair of adjacent primary electrode-s, a second auxiliary electrode disposed between a second pair of adjacent primary electrcdes, means for applying voltage to the auxiliary electrodes to create a voltage difference between them and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

8. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, at least four primary electrodes symmetric-ally disposed about a central axis, means for applying a DC. voltage to the primary electrodes to create a static multipole electric field component between the primary electrodes, means for applying an A.C. voltage to the primary electrodes to create an alternating multipole electric field component between the primary electrode-s, a first auxiliary electrode disposed between a first pair of adjacent primary electrodes, a second auxiliary electrode disposed between a second pair of adjacent primary electrodes and diametrically opposed to the first auxiliary electrode, means for applying voltage to the auxiliary electrodes to create a voltage difference between them and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

9. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-tocharge ratios, at least four primary electrodes symmetrically disposed about a central axis, means for applying a DC. voltage to the primary electrodes to create a static multipole electric field component between the primary electrodes, means for applying an AC. voltage to the primary electrodes to create an alternating multipole electric field component between the primary electrodes, a first auxiliary electrode disposed between a first pair of adjacent primary electrodes, a second auxiliary electrode disposed between a second pair of adjacent primary electrodes, means for applying equal but opposite voltages to the auxiliary electrodes to create a voltage difference between them and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-tocharge ratio are selectively detected.

10. A mass filter for selectively detecting charged particles comprising a source of charged particle having different mass-to-ch-arge ratios, at least four primary electrodes symmetrically disposed about a central axis, means for applying a DC. voltage to the primary electrodes to create a static multipole electric field component between the primary electrodes, means for applying an A.C. voltage to the primary electrodes to create an alternating multipole electric field component between the primary electrodes, a first auxiliary electrode disposed between a first pair of adjacent primary electrodes, a second auxiliary electrode disposed between a second pair of adjacent primary electrodes, means for applying A.C. voltage to the auxiliary electrodes to create an A.C. voltage difference between them and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

11. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, at least four primary electrodes symmetrically disposed about a central axis, means for applying a DC. voltage to the primary electrodes to create a static multipole electric field component between the primary electrodes, means for applying an A.C. voltage to the primary electrodes to create an alternating multipole electric field component between the primary electrodes, a first auxiliary electrode disposed between a first pair of adjacent primary electrodes, a second auxiliary electrode disposed between a second pair of adjacent primary electrodes, means for applying a DC. voltage to the auxiliary electrodes to create a DC. voltage difierence between them and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-tocharge ratio are selectively detected.

12. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, four primary electrodes symmetrically disposed about a central axis, at least two diametrically opposed auxiliary electrodes each disposed between a respective pair of primary electrodes, means for applying a positive DC. voltage of one value with reference to a common point to one pair of diametrically opposite primary electrodes and a negative DC. voltage of equal value with reference to the common point to another pair of diametrically opposed primary electrodes so as to create a static multipole electric field component between the primary electrodes, means for applying an A.C. voltage balanced about the common point to the opposite pairs of primary electrodes to create an alternating multipole electric field component between the primary electrodes, means for applying an electric voltage balanced about the common point to the pair of auxiliary electrodes and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of Zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

13. A mass filter for selectively detecting charged particles comprising a source of charged particles having different masstocharge ratios, four primary electrodes symmetrically disposed about a central axis, at least two diametrically opposed auxiliary electrodes each disposed between a respective pair of primary electrodes, means for applying a positive DC. voltage of one value with reference to a common point to one pair of diametrically opposite primary electrodes and a negative D.C. voltage of equal value with reference to the common point to another pair of diametrically opposed primary electrodes so as to create a static multipole electric field component between the primary electrodes, means for applying an AC. voltage balanced about the common point to the opposite pairs of primary electrodes to create an alternating multipole electric field component between the primary electrodes, means for applying an AC. voltage balanced about the common point to the pair of auxiliary electrodes and means for detecting charged particles passed through said filter in a direction substantially parallel to said electrodes, whereby a region of zero electrical field along the central axis is eliminated and only particles of a predetermined mass-to-charge ratio are selectively detected.

14. A mass filter for selectively detecting charged particles comprising a source of charged particles having different mass-to-charge ratios, four primary electrodes symmetrically disposed about a central axis, at least two diametrically opposed auxiliary electrodes each disposed between a respective pair of primary electrodes, means for applying a positive DC. voltage of one value with reference to a common point to one pair of diametrically opposite primary electrodes and a negative DC. voltage of equal value with reference to the common point to another pair of diametrically opposed primary electrodes so as to create a static multipole electric field component between the primary electrodes, means for applying an UNITED STATES PATENTS 2,642,535 6/1953 Schroeder 2504l.9 2,721,954 10/1955 Nygard 250-4l.9 2,896,083 7/1959 Hare et al. 25041.9 2,939,952 6/1960 Paul 2504l.9 2,950,389 8/1960 Paul 2504l.9 3,147,445 9/1964 Wuerker et al. 3304.7

FOREIGN PATENTS 888,913 2/1962 Great Britain.

RALPH G. NILSON, Primary Examiner.

25 H. s. MILLER, A. L. BIRCH, Assistant Examiners. 

1. A MULTIPOLE MASS FILTER FOR SELECTIVELY DETECTING CHARGED PARTICLES COMPRISING A SOURCE OF CHARGED PARTICLES HAVING DIFFERENT MASS-TO-CHARGE RATIOS, A PLURALITY OF SUBSTANTIALLY PARALLEL ELECTRODES SYMMETRICALLY DISPOSED ABOUT A CENTRAL AXIS, MEANS FOR APPLYING AN A.C. VOLTAGE TO THE ELECTRODES TO CREATE AN ALTERNATIONG MULTIPOLE ELECTRIC FIELD COMPONENT, MEANS FOR APPLYING A D.C. VOLTAGE TO THE ELECTRODES TO CREATE A STATIC MULTIPOLE FIELD COMPONENT, MEANS FOR PROVIDING A VOLTAGE DIFFERENCE BETWEEN AT LEAST TWO DIAMETRICALLY OPPOSITE ELECTRODES, AND MEANS FOR DETECTING CHARGED PARTICLES PASSED THROUGH SAID FILTER IN A DIRECTION SUBSTANTIALLY PARALLEL TO SAID ELECTRODES, WHEREBY A REGION OF ZERO ELECTRICAL FIELD ALONG THE CENTRAL AXIS IS ELEMINATED AND ONLY PARTICLES OF A PREDETERMINED MASSTO-CHARGE RATIO SELECTIVELY DETECTED. 